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In organic chemistry, Hückel's rule estimates whether a planar ring molecule will have aromatic properties. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931. It was first expressed succinctly as the 4n+2 (actually 2+4n) rule by von Doering in 1951. A cyclic ring molecule follows Hückel's rule when the number of its π electrons equals where is zero or any positive integer (although clearcut examples are really only established for values of n=0 up to about 6). Hückel's rule was based on calculations using the Hückel method, although it can also be justified by considering a particle in a ring system.